Process for initiating an arc between electrodes in a plasma gun



June 3, 1969 K. ARKLESS TAL 3,443,333

PROCESS FOR INITIATING AN A BETWEEN ELECTRODE? IN A PLASMA GUN Filed ma 51, 1966 Sheet 1 of 2 June ,1969 K. ARKLESS ETAL 3,448,333

PROCESS FOR INITIATING AN ARC BETWEEN ELECTRODES IN A PLASMA GUN United States Patent 01 U.S. c1. 315 111 11 Claims ABSTRACT OF THE DISCLOSURE A process for initiating an are between electrodes in a plasma gun is disclosed. A gaseous mixture is introduced between the electrodes, which gaseous mixture has a lower breakdown potential than its constituents. Generally the mixture comprises the primary gas to be heated and an additional gaseous constituent. As a result of this technique, the arc can be initiated at a lower potential than would have otherwise been possible.

The present invention relates to improved apparatus and processes for heating gases by means of an electric discharge between electrodes.

It is known to heat gases to high temperatures by passing the gas through an electric discharge formed between electrodes. One apparatus which utilizes this principle is the plasma gun which, in one form, consists of at least two electrodes connected to a suitable source of electrical energy, which may be a source of direct or alternating current, but which is preferably a source of direct current.

In one design of plasma gun there is a central solid electrode projecting toward the centre of a disc electrode. The disc electrode in such guns is usually provided with an orifice through the centre as an outlet for the gas which is heated by passage through the discharge between the electrodes and the arc is formed between the tip of the central electrode and the inner edge and/or the inner wall of the orifice through the other electrode.

It is preferred, at least when the electrodes are connected to a source of direct current, that the central electrode is the cathode.

The gas to be heated is generally introduced into such a gun around the central electrode either in helical flow (for example by the introduction of the gas tangentially to the wall of the chamber containing the central electrode) or in laminar flow (for example by the introduction of the gas through a number of orifices surrounding the central electrode). Alternatively a mixture of such flows may be provided.

Plasma guns of the type referred to above are described, for example, in our co-pending US. application Ser. No. 256,386, filed Feb. 5, 1963, and now abandoned.

Alternatively, the electrodes in the plasma gun may be tubes, preferably coaxial tubes placed in end to end relationship and the are being formed between the in teriors of the electrodes. In such guns the gas to be heated may be introduced into the chamber surrounding the adjacent ends of the electrodes, preferably through a number of inlets directed tangentially to the walls of the chamber to impart a helical flow to the gas before passing through the discharge.

After heating, the gas may conveniently be directed through the interior of one of the tubes forming an electrode to the atmosphere.

Alternatively, the gas to be heated may be introduced Patented June 3, 1969 through one tubular electrode and withdrawn through the other.

Any combination of these arrangements may also be used, as desired.

In all cases, due to the very high temperatures developed within the gun, it is generally advisable to provide adequate cooling of the plasma gun and particularly of the electrodes. This may be done by any convenient method but it is preferred to do so by circulating a coolant, for example water, through passages within the electrodes and/or in the body of the gun under pressure. If desired, the resulting heated coolant may then be circulated in heat exchange relationship with the incoming gas to be heated, thereby conserving heat.

By the term plasma gun as used in this specification, therefore, is meant a device whereby a gas is heated to a high temperature by its passage through an electric are between electrodes.

Plasma guns are generally designed to operate in a stable manner once the arc has been initiated but the initiation of the arc presents considerable practical difficulties because the voltage required to initiate the arc is much greater than that required to maintain the are once the latter is established. The voltage required to initiate the are, therefore, must exceed the breakdown potential of the gas between the electrodes. Power units can be designed to provide the high voltages necessary to initiate the are (as well as to maintain the are) but this greatly increases the cost of the power unit.

Because of this, various other methods have been used to initiate the are, for example the provision of auxiliary circuits to provide high frequency discharges of short duration between the electrodes. Such auxiliary circuits require careful construction and, again, considerably increase the cost of the installation. Furthermore, there is a danger that the high frequency discharge may damage the main power unit connected to the electrodes.

Another method of initiating the arc in plasma guns which has been used is the insertion between the electrodes of a metal wire or the like whereby the metal is rapidly heated and thus produces ions which assist in providing a path for the are which may then become established. A variation of this method is the provision of a third electrode, for example a carbon electrode which is in electrical contact with one electrode and which is momentarily advanced, for example pneumatically, until it contacts the other electrode thereby completing the circuit. This short circuit initiates the are by a temporary reduction of the gap between the electrodes, thus increasing the potential gradient between the electrodes until it exceeds the breakdown potential of the gas.

These methods (and other methods which have also been used) are cumbersome and possess many disadvantages. The use of a heated wire, for example, gives rise to contamination of the gas passing through the gun and may weld itself to the electrode thereby causing arc instability.

The use of a carbon electrode to produce a momentary short circuit between electrodes may also give rise to high currents in the circuit, which may damage the power supply. The carbon electrodes have also been found to be unreliable; to restrict gun design and to undergo very rapid erosion necessitating frequent replacement (which entails interruption of the guns operation).

Unreliability in the operation of a plasma gun is a particularly serious disadvantage when the plasma gun forms part of a complex chemical plant which relies upon the gun as a source of heat.

It is an object of the present invention to provide a process and a means for initiating the are within a plasma gun which eliminates some or all of the above disadvantages.

Accordingly, the invention is a process for initiating an are between electrodes comprising passing a primary gas to be heated between electrodes connected to a suitable source of electrical energy and introducing, between the electrodes, a secondary gas or gas mixture in such a manner that it forms, either when mixed with the primary gas or by itself, a gaseous mixture between the electrodes which has a lower breakdown potential than that of its constituents and of the potential drop between the electrodes.

The invention is also a plasma gun for heating a gas to a high temperature comprising at least two electrodes adapted for connection to a source of electrical energy; at least one inlet for the gas to be heated by passage between the electrodes and an outlet for the heated gas, characterized in that there is provided a conduit, the wall of which is in electrical contact with one electrode and which is so positioned as to direct a gas introduced through the conduit between the surfaces of the electrodes and means to introduce a gas into the said conduit.

The gas mixture which has a lower breakdown potential than that of its constituents may be formed either by introducing a single secondary gas into the space between the electrodes in such a manner that it mixes with the primary gas to be heated, passing between the electrodes, and forms with this gas a mixture of the correct proportions to obtain the desired low breakdown potential, or by introducing a secondary gas mixture, for example a preformed mixture between the electrodes.

It will be appreciated that where the secondary gas or gas mixture is introduced into a space between electrodes through which the primary gas to be heated is already passing, the gases will mingle to give concentration gradients which may be considered in the form of a large number of gas mixtures containing various concentrations of their constituent gases between the electrodes and, in the present invention, the secondary gas or gas mixture is introduced into the plasma gun in such a manner that at least one resulting gas mixture exists between the electrodes containing the correct proportion of gases to give the desired low breakdown potential.

The secondary gas or gas mixture which is introduced between the electrodes is preferably introduced through a conduit, the wall of which is in electrical contact with one of the electrodes, and which directs the gas or gas mixture at the face of the other electrode. One particular method of producing such an arrangement is by drilling a hole through the wall of the gun, through a portion of one electrode and at such an angle that gas issuing from the interior end of the conduit is directed upon the face of another electrode between which the are is to be formed. If desired, a tube or the like may be inserted in the hole to facilitate variation of the velocity and/or direction of the gas flow. The distal end of the conduit is normally provided with means to introduce the secondary gas or gas mixture into the conduit, for example this may comprise a connection to a gas supply pipe.

Alternatively, the wall of the conduit for the secondary gas or gas mixture may not be in electrical contact with an electrode and so positioned that the second gas or gas mixture is directed between the faces of the electrodes.

The conduit may be of any suitable cross section and may, for example, terminate in the gun in a slot, annulus or porous plug. Alternatively, there may be a number of such conduits or even a single conduit which may terminate in a number of orifices of the type mentioned above.

The end of the conduit should preferably not project into the arc (when initiated) in order to avoid excessive erosion of the conduit. However, it may be advantageous, in certain circumstances, to momentarily advance the conduit into the space between the electrodes (preferably without short circuiting them) and to withdraw it immediately after the arc is initiated. By such means it may be possible to use a conduit of smaller internal diameter (particularly where the space between the electrodes is large) and thereby reducing the amount of secondary gas or gas mixture required to initiate the arc.

The optimum diameter of the conduit will depend, inter alia, upon: the number of conduits; the distance between the electrodes; the composition of the gas mixture required to give a low breakdown potential; the amount of gas to be introduced between the electrodes and the pressure available for its introduction. Generally, however, a conduit or conduits having an internal diameter in the range of about /2 to and particularly in the range of to have been found convenient, although larger diameter conduits may be desirable, particularly where the electrode gap is large.

The secondary gas or gas mixture which is introduced into the space between the electrodes (between which the arc is to be initiated) in order to reduce the breakdown potential must be such a nature that, if it is a single gas, it forms with the primary gas, a gaseous mixture having a lower breakdown potential than that of either of the constituents of the mixture and, if a gas mixture, then that mixture of gasses must be such that it provides a lower breakdown potential between the electrodes than any of the constituents of the mixture or of the primary gas.

It is believed that the process of the present invention depends upon the formation of a zone or zones of a gas mixture between the electrodes which is characterised by the presence of a major gaseous constituent having a high metastable potential (for example the secondary gas or gas mixture) and a minor constituent(s) having a lower ionization potential (for example the primary gas or a constituent of the secondary gas mixture) than the metastable potential of the major constituent.

The voltage applied across the electrodes is believed to give rise to metastable molecules of the major constituent (which have a relatively long life when compared with ionized molecules) and these interact with molecules of the minor constituent thereby causing their ionization. The ionization of the molecules of the minor constituent is believed to be maintained by their high dilution and/or by further ionization due to their interaction with the metastable molecules of the major constituent, thus creating a pathway for the flow of current between the electrodes which rapidly increases to form a self-sustaining are. This are is then transferred to the primary gas, and the flow of secondary gas or gas mixture is discontinued.

Gases which have a high metastable potential (and which are therefore particularly suitable as the secondary gas) are helium (19.8 electron volts) and neon (16.6 electron volts). Argon, which is less suitable, has a metastable potential of 11.5 volts.

Primary gases (or gases which may be introduced through the conduit as a component of a secondary gas mixture) and which are very suitable for use in the present invention as ionizing gases (particularly with helium or neon as the secondary gas) are hydrogen, oxygen, nitrogen, argon (where this is not used as the secondary gas), chlorine, carbon monoxide, carbon dioxide, ammonia, hydrogen chloride, nitrogen oxides, sulphur dioxide, and methane which have ionization potentials in the range of about 10 to 15 electron volts.

The local concentration of the secondary gas between the electrodes, for example helium or neon, should be greater than and preferably greater than by volume to obtain the minimum breakdown potential, the ionizing gas or gases forming the remainder of the mixture.

As an example of one manner in which the present invention may be carried out a primary gas such as nitrogen may be passed between the electrodes to which electrical power is supplied and helium or neon, as the an orifice diameter) at 50 p.s.i.g. and the introduction of this gas mixture was controlled by a solenoid valve.

When the power supply was set at 1500 volts, momentary operation of the solenoid valve resulted in the immediate initiation of the arc and this continued after the supply of the helium/argon gas mixture was discontinued.

Example 3 The process described in Example 2 was repeated except that oxygen was introduced as the primary gas in place of nitrogen.

It was again found that no arc could be initiated at 1500 or 3000 volts.

An arc was readily initiated, however, when helium (as the secondary gas) was introduced between the electrodes at 1500 volts (open circuit voltage) and was readily maintained after the helium supply was discontinued.

Example 4 The process of Example 2 was repeated using helium neon and argon separately as primary gases without the introduction of a secondary gas. In no case could the are be initiated either at 1500 or at 3000 volts open circuit voltage.

What is claimed is:

1. In an arc plasma process wherein a primary gas is passed at high velocity through an electric are between electrodes and is heated by said passage through said are to the plasma state, the improved method of initiating said arc which comprises:

(a) establishing a potential difference between said electrodes which is less than the breakdown potential of said primary gas but is above the minimum necessary to maintain an established arc in said primary (b) passing between said electrodes a gas mixture having a lower breakdown potential than that of any of the mixture components and lower than the potential difference between said electrodes whereby an arc is initiated between said electrodes; and

(c) transferring said are to said primary gas.

2. A process in accordance with claim 1 wherein said gas mixture in step (b) is formed by mixing a substantial amount of at least one secondary gas with said primary gas and wherein said are transfer in step (c) is effected by discontinuing the mixing of secondary gas with said primary gas.

3. A process as claimed in claim 2 wherein the primary gas is oxygen and the secondary gas comprises helium.

4. A process as claimed in claim 2 wherein the primary gas is selected from the group consisting of hydrogen, p

nitrogen, chlorine, carbon dioxide, carbon monoxide, ammonia, hydrogen chloride, nitrogen oxides, sulphur dioxide, methane and argon and mixtures thereof, and the secondary gas is a gas other than said primary gas and is selected from the group consisting of helium, neon, argon and mixtures thereof.

5. A process as claimed in claim 2 wherein the total concentration of secondary gas in the gas mixture between the electrodes comprises at least by volume of the gas mixture.

6. A process as claimed in claim 5 wherein the concentration of secondary gas comprises at least by volume of the mixture.

7. A process as claimed in claim 1 wherein the primary and secondary gases are introduced in a manner to form a gas mixture between the electrodes which contains a major proportion of gas having a high metastable potential and a minor proportion of gas having a lower ionization potential than the metastable potential of the gas forming the major proportion of the mixture.

8. A process as claimed in claim 7 wherein the ionization potential of gas forming the minor proportion of the gas mixture between the electrodes is in the range 10 to 15 electron vol-ts.

9. A process as claimed in claim 2 wherein secondary gas is introduced between the electrodes at a pressure in the range 5 to 500 p.s.i.g. above that existing between the electrodes.

10. A process in accordance with claim 1 wherein said gas mixture in step (b) is a mixture of at least two gases other than said primary gas and wherein said are transfer is efiected by feeding primary gas between said electrodes through said arc, and discontinuing feeding of said gas mixture.

11. A process in accordance with claim 10 wherein said gas mixture in step (:b) comprises a mixture of at least one member selected from the group consisting of helium and neon with an ionizing gas.

References Cited UNITED STATES PATENTS 1/1966 Carlson et a1. 315-111 6/1966 Miller 313-231 X JAMES W. LAWRENCE, Primary Examiner.

PALMER C. DEMEO, Assistant Examiner.

U.S. Cl. X.R. 

